In the course of assessing and producing hydrocarbon bearing formation and reservoirs, it is helpful to acquire knowledge of formation and/or formation fluid properties which influence the productivity and yield from the formation drilled. Such knowledge might be acquired by methods referred to as “logging.”
In general, logging operations involve the measurement of a formation parameter or of a formation fluid parameter as a function of location or, more specifically, as a function of borehole depth. Formation logging has evolved to encompass many different types of measurements including measurements based on sonic, electro-magnetic or resistivity measurements and nuclear measurements such as nuclear magnetic resonance or neutron capture or scattering effects.
It is further well established to mount the measurement tools for a logging operation on either dedicated conveyance means such as wireline cables, drill pipe or coiled tubing (CT) or on the drilling string. The latter case is known in the industry as measurement-while-drilling (MWD) or logging-while-drilling (LWD). In MWD and LWD operations the parameter of interest is measured by instruments that might be mounted close behind the bit or the bottom-hole assembly (BHA). Both, logging in general and LWD are techniques known for decades and hence are believed to require no further introduction.
Also known for as many as three decades are measurements broadly referred to as log-inject-log or LIL measurements. From their inception as exemplified by the U.S. Pat. No. 3,562,523 issued to Richardson and Wyman, LIL based methods have evolved into many variants.
However, the basic principle of LIL can still be seen as including a sequence starting with using a first drilling fluid system, which invades the formation over a time period and then logging the invaded formation for a desired parameter. Subsequently, the composition of the drilling fluid might be changed and this new fluid is circulated. After the new fluid has invaded the rock formation, the logging operation is repeated. The second measurement thus registers the change in the value or spatial distribution of parameter in question after a specified or an arbitrary elapsed time.
Known variants of the LIL methods are described for example in the U.S. Pat. No. 3,748,474 to Murphy, the U.S. patent RE 28,963 to Fertl and Reynolds and in the U.S. Pat. No. 4,987,368 to Vinegar. Further reference to LIL can be found in the U.S. Pat. No. 5,796,252 to Kleinberg et al. The latter document provides further insight into and details of state-of-the-art NMR logging.
In view of the above known art, it is proposed to present a method of logging a formation which improves upon the fixed elapsed time method and solves problems associated with the known log-inject-log methods.